There are known conventional electrodes for use in electrolysis that include a conductive substrate and a catalyst layer with which the conductive substrate is coated. Known methods for manufacturing such electrodes for electrolysis include subjecting a conductive substrate to sand blasting or acid etching for surface roughening so that a catalyst layer can be deposited with improved adhesion onto the surface of the conductive substrate; and then forming a catalyst layer on the roughened surface of the conductive substrate (see, for example, Patent Documents 1 and 2).
An aqueous solution of an alkali metal salt, specifically, an aqueous solution of sodium chloride is electrolyzed to produce chlorine, hydrogen, and sodium hydroxide. It is well known that this process is performed using a brine electrolyzer for ion-exchange membrane process, which includes an anode chamber and a cathode chamber separated by a cation-exchange membrane and is configured to allow a current to flow between an anode in the anode chamber and a cathode in the cathode chamber so that electrolysis is performed. There have been various modifications of this type of electrolyzer. For example, dimensionally stable electrodes are developed as anodes, and active cathodes with low hydrogen overpotential are developed as cathodes, so that electrolytic voltage for brine electrolysis using ion-exchange membrane process is reduced. Particularly, recent improvements in electrolysis technology are remarkable. One of such improvements is a zero-gap brine electrolyzer having an anode and a cathode both in tight contact with a cation-exchange membrane, which is developed to further reduce electrolytic voltage (see, for example, Patent Documents 3 and 4).
In brine electrolyzers for ion-exchange membrane process, the anode is inherently in tight contact with the ion-exchange membrane. In zero-gap brine electrolyzers, the cathode is additionally brought into tight contact with the ion-exchange membrane. The ion-exchange membrane is naturally pressed against and brought into tight contact with the anode because the liquid pressure is higher on the cathode side than on the anode side so that the electrolyte pressure differs between the anode-side and cathode-side of the ion-exchange membrane. In addition to this state, zero-gap brine electrolyzers are designed in such a way that the cathode is intentionally and physically brought into tight contact with the ion-exchange membrane so that the electric resistance between the ion-exchange membrane and the cathode can be reduced and thus the electrolytic voltage can be reduced. In such zero-gap brine electrolyzers, the pressure at which the ion-exchange membrane is pressed against the anode increases as the cathode is brought into tight contact with the ion-exchange membrane.
To address the increase in the pressing pressure, the zero-gap brine electrolyzer described in Patent Document 4 is designed in such a way that its anode has a rigid structure with rigidity high enough to be less deformable even when pressed against an ion-exchange membrane, while its cathode has a flexible structure that can maintain the zero-gap by absorbing irregularities caused by the tolerances and deformation of its electrode support frame and other components. In addition, a conductive cushion mat is provided between its cathode and a back board so that tight contact between the ion-exchange membrane and the anode and between the ion-exchange membrane and the cathode can be ensured without damaging the ion-exchange membrane. Patent Document 4 also recommends a rigid structure of the anode in which, mainly to ensure liquid permeability between the anode and the ion-exchange membrane, a catalyst layer should be formed on the surface of a conductive substrate made of an expanded metal of titanium or a mesh of titanium, and the maximum height difference of the surface irregularities of the catalyst layer should be from 5 to 50 μm.